According to the criteria issued by WHO (World Health Organization) based on a glucose tolerance test, diabetes mellitus and impaired glucose tolerance (hereinafter sometimes referred to as IGT) are distinguished by the fasting blood glucose level and the blood glucose level 2 hours after glucose loading. Patients with IGT have high blood glucose levels compared to those of patients with diabetes mellitus, and are reported to be at increased risk of developing diabetes mellitus and complications of arteriosclerotic diseases. In particular, it is known that patients with IGT who have blood glucose levels of 170 mg/dl or above at 2 hours following glucose loading, i.e., patients with high-risk IGT, may develop diabetes mellitus at a high rate [Diabetes Frontier, p. 136, 1992].
With regard to voglibose which is an .alpha.-glucosidase inhibitor, there are reports of studies on effects of voglibose for insulin-resistant IGT and diabetes [Yakuri-to-Chiryo (Japanese Pharmacology & Therapeutics), 24 (5):213 (1996); Metabol. Exp. Clin., 45:731, 1996].
Voglibose (AO-128) is also known to have effects of lowering blood glucose level and improving glucose tolerance in rats [Yakuri-to-Chiryo (Japanese Pharmacology & Therapeutics), 19 (11):161 (1991); Journal of Nutrition Science and Vitaminology, 45 (1): 33 (1992)]. On the contrary, it has also been reported that the effect of voglibose in improving glucose tolerance could not be verified in human [Rinsho-Seijinbyo, 22 (4): 109 (1992)].
For the above-mentioned high-risk IGT, in particular, there have been no report so far about research for preventing its transition to diabetes mellitus or about its treatment to recover the normal condition.